THIS INVENTION relates to a process and installation for the treatment of solid material by means of an endothermic chemical reaction. More particularly, the invention relates to a process for the treatment of a solid material such as a mineral to cause it to undergo an endothermic chemical reaction, and to an installation for the treatment of such solid material undergoing said endothermic reaction, the process and installation being suitable for, but not limited to, the treatment of a mineral at elevated temperatures at which the mineral being treated become sticky and/or soft. The invention also relates to a kiln forming part of the installation.
The Applicant is aware of the abstract of Japanese Published Patent Application 56166155, published under publication number JP-A-58067813, which abstract has been published in Patent Abstracts of Japan, Volume 7, No. 155 (C-175), Jul. 7, 1983. This abstract discloses a process for the treatment by reduction and sintering of a solid material by passing the solid material along the inside of a tunnel kiln comprising a horizontally extending tunnel having a hollow interior. The solid material is supported on a series of supports as it passes along the kiln, the supports being moved successively along the interior of the kiln. The solid material is heated, by means of radiant heat radiated on the solid material, as it passes along the kiln, to a temperature at which it undergoes an endothermic reaction. A fuel such as coke oven gas is supplied to a burner in the roof of the kiln, so that heat is produced by combustion in a combustion zone in the upper part of the kiln, separate from a reaction zone where reduction and sintering of the solid material is carried out, in the lower part of the kiln. The Applicant is also aware of U.S. Pat. No. 4,978,294 which discloses a process whereby, in a rotary furnace, partitions are used to keep combustion gases separate from minerals being reduced, so that re-oxidation of the minerals is resisted. The Applicant is further aware of published International Patent Application WO-A-93/16342 which discloses shapes in the form of extruded pipes consolidated from particles of solid material, which are stacked on supports during heating thereof.
According to one aspect of the invention there is provided a process for the treatment of a solid material, the process including the process steps of:
passing the solid material along the inside of a kiln comprising a horizontally extending tunnel having a hollow interior;
supporting the solid material on a succession of supports as it passes along the kiln, the supports being moved successively along the interior of the kiln; and
heating the solid material, by means of radiant heat radiated on to the solid material, as it passes along the kiln, to a temperature at which the solid material undergoes an endothermic chemical reaction,
the heat which is radiated on to the solid material being produced by combustion in a combustion zone separated by at least one member of the group consisting of partitions, panels and baffles from a reaction zone through which the solid material supported on the supports passes during the heating.
According to another aspect of the invention there is provided a process for the treatment of solid material, the process including the steps of:
passing the solid material along the inside of a kiln comprising a horizontally extending tunnel having a hollow interior;
supporting the solid material on a succession of supports as it passes along the kiln, the supports being moved successively along the interior of the kiln; and
heating the solid material, by means of radiant heat radiated on to the solid material, as it passes along the kiln, to a temperature at which the solid material undergoes an endothermic chemical reaction,
the process including the step of consolidating particles of the solid material into shapes which are arranged in stacks on the supports.
Each support may be in the form of a wheeled trolley, the process including loading a succession of the trolleys with the solid material to be treated, each trolley being loaded on an upwardly facing support surface of a load bed of the trolley, the moving of the supports along the interior of the kiln being by rolling the loaded trolleys in succession along a path extending, below the interior of the kiln, along the length of the kiln,
The kiln may have an inlet end and an outlet end, each of which ends is provided with an airlock, the process including the steps of inserting the loaded trolleys in succession into the inlet end of the kiln, and withdrawing the loaded trolleys in succession from the outlet end of the kiln, the airlocks acting to promote the maintenance of an atmosphere inside the kiln which is different from the ambient atmosphere outside the kiln, which atmosphere inside the kiln promotes the endothermic reaction.
Heating the mineral may be by radiant heat provided in a reaction zone in the tunnel by heating surfaces of electric heating elements in the tunnel. However, heating the mineral is preferably by radiant heat emitted by one or more heating surfaces facing towards the mineral on the trolleys in said reaction zone in the interior of the tunnel, the heating surfaces being heated by a combustion gas and being provided by one or more partitions in the interior of the tunnel and the combustion taking place on the side of each partition remote from the mineral on the trolleys. In other words, the heating of the solid material may be by radiant heat emitted by one or more heating surfaces in the interior of the kiln and facing towards the solid material passing along the kiln, each heating surface being provided by a partition in the interior of the kiln and each partition having opposite sides facing respectively towards and away from the solid material, each partition being heated by a combustion gas located on the side of the partition facing away from the solid material. Instead, heating may be by radiation from a flame created by combustion of a gas.
The process may include the step of consolidating particles of the solid material into shapes to promote heating thereof in the kiln, eg by both convective and radiant heating, the shapes being stacked on the supports and the process including the step of removing from the vicinity of the shapes any gaseous products formed by the heating of the shapes, eg formed by the endothermic reaction and which can inhibit continuance of such reaction. The process may, accordingly, include the step of stacking consolidated shapes on the trolleys. Instead or in addition, the solid material or mineral to be heated may be loaded on trays, the trays in turn being loaded in spaced positions on the trolleys, each trolley carrying a plurality of trays. When consolidated shapes are employed, they may be in the form of extrusions or compacted mouldings, the solid material being milled prior to its being extruded or moulded and optionally being mixed with one or more constituents selected from reagents such as reductants which participate in the endothermic reaction, selected from catalysts or fluxes which can enhance the endothermic reaction, and selected from binders for facilitating the consolidation.
The solid material may, in the interior of the tunnel and prior to the radiant heating thereof to cause the endothermic reaction, be subjected to pre-heating. The pre-heating may be by radiant heating, eg similar to the heating in the reaction zone, or preferably by convective heating, for example by forced convection achieved by circulating a hot gas transversely through the interior of the tunnel and over the solid material on the trolleys. The hot gas may be heated by a heat exchanger, or it may be a hot combustion gas. In the interior of the tunnel and after the endothermic reaction, the reaction product formed by the endothermic reaction may be cooled by conveying the reaction product along a cooling zone in the interior of the tunnel, prior to withdrawal of the trolleys from the tunnel. In the case of reducing reactions, gas produced as a by-product of an endothermic reducing reaction may be withdrawn from the vicinity of the solid material or of its reaction product, and may be burnt to form the combustion gas which heats the panels of the reaction zone.
In particular, the mineral to be heated may comprise particles consolidated into chevron shapes made up of two flat slabs intersecting at a corner, being stackable on the edges of the slabs in stable fashion on a flat load bed of a trolley, with the shapes arranged in a spaced roughly nesting arrangement which permits radiant heating of the slab faces from above and gas flow over the slab faces from either side of the trolley to the other. Instead, the shapes may be in the form of hollow cubes or blocks having openings into hollow interiors via at least three faces thereof, to permit, when they are stacked on trolleys, radiation to enter their interiors from above, while permitting gas to pass through their interiors from either side of the trolley to the other. The nature of the shapes and the thickness of the material thereof may be chosen to promote one or more of good heat transfer to the shapes, good diffusion of reactive gases into the shapes, good strength of the shapes and good dimensional stability of the shapes.
A further feature of the process of the invention is the possibility of producing a reduced product of a shape and/or size which can be employed in a subsequent processing step without the necessity of any size reduction thereof such as milling thereof. Thus, shapes of small size and/or low wall thickness may be used, capable of being fed directly to a subsequent smelting step, without size reduction. In such cases, when the next step to which the mineral will be subjected may be smelting, the process contemplates transferring the consolidated shapes, after the endothermic reaction, in a hot state, without cooling, to the smelting step or the like step.
When the endothermic reaction is a reduction of the solid material or mineral, a solid or liquid reductant, which may be carbonaceous, may be mixed with the mineral to be reduced, or a gaseous reductant, which may be hydrogen or may be carbon-containing, may form part of the gas passed over the mineral on the trolleys. Thus, a solid reductant, such as coal or char, or a liquid reductant such as tar, may be included as constituent of consolidated shapes stacked on the trolleys; or a liquid such as fuel oil may be mixed with the mineral held in trays stacked on the trolleys. When the reductant is part of the gas circulated over the mineral, it may be hydrogen or a hydrocarbon gas such as methane, or it may be carbon monoxide, or the like.
In particular, when the endothermic reaction is a reduction, the process may include the step of admixing the particles of solid material, before consolidation thereof, with a carbon-containing reductant, the consolidation being into shapes of a size such that the process produces a product in the form of shapes of reduced solid material which can subsequently be smelted without any size reduction prior to the smelting thereof. As indicated above, the process may include the step, prior to the radiant heating thereof to cause the endothermic reaction, of pre-heating the solid material, and includes, after said radiant heating, the step of cooling the solid material.
In a particular embodiment, hot gas from the reaction zone may be used for the pre-heating. This gas may initially be too hot for circulation by fans, being eg at 1600xc2x0 C. or more, whereas fans are preferably operated at below eg 900xc2x0 C. In this case the hot gas may be diluted with air to lower its temperature before it passes over the fan or fans. If this dilution oxidizes carbon monoxide fully to carbon dioxide in the hot gas by reaction of oxygen in the air with carbon monoxide in the hot gas, the carbon dioxide produced may react unacceptably or undesirably with any carbonaceous reductant in the consolidated mineral, rendering the hot gas unsuitable for passing over the mineral. Similarly, if sufficient excess air is added to lower the hot gas temperature for there to be oxygen present in the cooled diluted gas, it can react undesirably with said carbonaceous reductant. In such cases the gas from the reaction zone may be used, via a heat exchanger, to heat a reducing gas with suitable reducing properties, which reducing gas is circulated over the mineral by one or more fans. Instead, baffles in the tunnel on opposite sides of the trolley track may be used to direct hot gas from the reaction zone in zig-zag fashion across mineral on a train of trolleys on the track, in an upstream direction relative to trolley movement away from the reaction zone, gas flow being caused by an extraction fan for withdrawing gas from the tunnel, and the gas being cooled by heat exchange with the mineral on the trolleys moving countercurrently to the gas, the mineral being heated by the gas.
When combustible volatiles are formed from carbonaceous reductants during the pre-heating step, it may be preferred to withdraw gases from the pre-heating step into the reaction zone for combustion thereof there to form combustion gases for heating the reaction zone. Instead, such volatiles may be removed from exhaust gases from the preheating at a position where they are sufficiently hot for addition of air thereto to cause complete combustion of the volatiles.
When, as indicated above, radiation from one or more heating surfaces heated by combustion gases is used to heat the mineral, the heating surfaces may be located alongside the track, eg on opposite sides of a train of trolleys on the track, and/or a heating surface may be located above the trolleys. This acts to separate the reaction zone from the combustion zone in which the combustion gases are produced. If a said heating surface is provided by a panel or wall made of a refractory membrane such as a silicon carbide membrane, cracking or breaking of the membrane can lead to undesirable downtime. A possibility contemplated by the present invention is thus to provide each trolley with its own heating surface or surfaces. Thus, each trolley may be provided with a roof and/or walls, eg as a box of refractory membrane, more or less enclosing the mineral on the trolley and separating it from combustion gases. In particular, a panel of such membrane may be loosely and removably placed on top of the mineral stacked on the trolley. Such panels can be kept from sticking to the mineral by means of carbon layers, provided eg by layers of coal or sawdust, spread on the mineral below the panels. The panels can be removed from trolleys which have left the tunnel and recycled to the tunnel entrance for re-use.
In other words, the process may include the step of heating the solid material on the supports by means of a plurality of heating membranes, one for each support, each heating membrane being supported on one of the supports and passing along the inside of the kiln on said support, the heating membranes radiating the radiant heat on to the solid material on said support and the process including using each membrane to heat solid material in turn on each of a plurality of the supports passing along the kiln.
Instead of using a panel or membrane to keep carbon dioxide or oxygen away from the mineral, a sufficient rate of carbon monoxide evolution in the mineral in the reaction zone may prevent or acceptably reduce carbon dioxide flow or diffusion towards and/or into the mineral Using excess reductant in the mineral can assist this and can confine any reoxidation of reduced mineral by carbon dioxide to the surface regions of consolidated mineral shapes. The geometry of the consolidated shapes, and their arrangement and spacing on the trolleys, may also be selected to resist flow or diffusion of carbon dioxide towards the surfaces of the shapes. Lowering gas velocities of the combustion gases above the trolleys, and the provision of suitable baffles, can also be employed to resist such reoxidation of reduced mineral by carbon dioxide from the combustion gases. These baffles can be part of the tunnel or can be mounted on the trolleys above the material.
The process of the invention may further involve the pre-heating of any air or oxygen used to form combustion gases for heating the reaction zone. This pre-heating can be by means of a heat exchanger heated by combustion gases which have been used to heat the reaction zone.
According to another aspect of the invention there is provided an installation for the treatment of solid material undergoing an endothermic chemical reaction, the installation including:
a kiln in the form of a horizontally extending tunnel having a hollow interior with an inlet end and an outlet end, the tunnel having a roof, a floor and a pair of opposed side walls;
a path for supports loaded with the solid material to pass along in the interior of the tunnel in succession, from the inlet end of the kiln to the outlet end thereof, the path extending along the floor at the bottom of the interior of the tunnel from the inlet end of the kiln to the outlet end thereof;
one or more heating surfaces for radiating radiant heat towards solid material loaded on supports passing along the path from said inlet end to said outlet end; and
a plurality of supports, movable in succession along the path from the inlet end of the kiln to the outlet end thereof,
the kiln having a reaction chamber in the interior of the tunnel which is separated from a combustion chamber in the interior of the tunnel by at least one member of the group consisting of partitions, panels and baffles, the floor of the tunnel providing a floor for the reaction chamber along which reaction chamber floor the path for the supports extends.
The supports may be in the form of wheeled trolleys, the path being in the form of a track comprising a pair of spaced rails for supporting the wheels of the trolleys.
The inlet end and the outlet end of the tunnel may each be provided with an airlock, for example a double-door chamber capable of receiving a support, the chamber having an inner door leading into the interior of the tunnel, and an outer door leading to the exterior of the kiln, the doors of each airlock being arranged so that, when the inner door is open, the associated outer door is closed, and so that, when the outer door is open, the associated inner door is closed. In other words, the installation may include an inlet airlock into the kiln at the inlet end of the kiln, and an outlet airlock out of the kiln at the outlet end of the kiln, for promoting isolation of an atmosphere in the interior of the kiln from the ambient atmosphere outside the kiln.
The tunnel may have its roof, side walls and floor made of a refractory material which preferably has heat-insulating properties to resist heat loss from the interior of the kiln. In a particular construction the track or path may be in the form of a channel extending along the length of the floor of the tunnel, midway between the side walls, for receiving the supports such as trolleys, the channel optionally having a pair of spaced rails extending along its length for supporting the wheels of the trolleys, each support having an upwardly facing load bed at the same height as the floor of the tunnel. Each trolley may thus have a load bed, conveniently flat, horizontal and upwardly facing, of a refractory material which preferably has heat-insulating properties, its load bed registering with the floor of the tunnel and preferably fitting with a close operating clearance between opposed parts of the floor on opposite sides of the channel.
The tunnel may have a reaction zone in which the heating surface or surfaces are provided. While these heating surfaces may be provided by electric heating elements, in a particular construction of the kiln the reaction zone is provided by part of the interior of the tunnel, which is divided by a pair of longitudinally extending panels or partitions into three longitudinally extending chambers, the partitions reaching upwardly from the floor of the tunnel, on opposite sides of the channel, to the roof of the tunnel, and dividing the interior of the tunnel into a central longitudinally extending reaction chamber along the floor of which the channel extends, and, on opposite sides of the reaction chamber, a pair of longitudinally extending combustion chambers defining combustion zones. In a development of this feature, a combustion zone in a combustion chamber may be provided in similar fashion above the reaction chamber and extending along the length of the reaction chamber, a panel or partition above the reaction chamber separating it from this combustion chamber and radiating heat downwardly into and on to the shapes or particulate reaction mixture. Any shapes and stacking arrangement used may thus be selected to facilitate radiant heating of the mineral from above. In general, thus, each heating surface may be provided by a longitudinal partition extending longitudinally along the interior of the tunnel and separating a reaction chamber in the interior of the tunnel from a combustion chamber in the interior of the tunnel, the floor of the tunnel providing a floor for the reaction chamber along which reaction chamber floor the path for the supports extends.
Instead, the heating surface may be provided by the interior surface of the roof of the tunnel, the tunnel being provided, in the reaction zone, with a plurality of longitudinally spaced baffles in the form of transverse partitions extending between the side walls, the baffles being spaced below the roof of the tunnel and spaced above the floor of the tunnel.
The tunnel may have a heating zone, upstream of the reaction zone and between the reaction zone and the air lock at the inlet end of the kiln; and the kiln may have a cooling zone, downstream of the reaction zone and between the reaction zone and the airlock at the outlet end of the kiln, the heating zone and cooling zone respectively being in communication with the reaction chamber of the reaction zone. In other words, the kiln may have a heating zone between the reaction zone and the inlet end of the kiln and a cooling zone between the reaction zone and the outlet end of the kiln, the heating zone and the cooling zone respectively being in communication with opposite ends of the reaction zone, and the path extending along the floor of the tunnel in the heating zone and in the cooling zone. The cooling zone may have one or more gas outlets feeding into the combustion chambers of the reaction zone; and the heating zone may be provided with a heating circuit, the heating circuit comprising hot gas circulation means such as a blower or, preferably, a fan, and/or with a gas heater such as a burner or heat exchanger, the circuit being arranged to convey hot gas from the heater to the heating zone of the kiln and to circulate it transversely through the heating zone, from one side of the heating zone to the other, and over mineral on the trolleys passing along the heating zone, to pre-heat the mineral before it enters the reaction chamber. There may be a plurality of such heating circuits, spaced in series along the length of the heating zone.
In a particular construction of the kiln, the cooling zone may have a pair of gas outlets feeding respectively into the downstream ends of the combustion chambers of the reaction zone, the combustion chambers each having a plurality of air inlets spaced in series along the length of the combustion chambers; and each combustion chamber may have a combustion gas outlet at its upstream end. Each of the heating zone and the cooling zone may be provided with one or more baffles or partitions reaching upwardly from the floor to the roof of the tunnel, and extending from the side walls of the tunnel, and across the floor of the tunnel, up to the edges of the channel in the floor of the tunnel, to resist gas flow longitudinally along the tunnel, on opposite sides of the trolleys in the heating zone and cooling zone; and similar partitions or baffles may be provided at opposite ends of the reaction chamber, to resist gas flow longitudinally into or out of the reaction chamber. Generally, thus, the tunnel may have, in its interior, a plurality of transverse partitions on each side of the path, the partitions resisting gas flow along the tunnel on opposite sides of the path in the heating zone and in the cooling zone, and the partitions resisting gas flow along the kiln on opposite sides of the path, into and out of the reaction zone.
In a further particular construction of the kiln, it may be provided with partitions or baffles on opposite sides of the track in the heating zone, and an extraction fan at the trolley inlet end of the heating zone, remote from the reaction zone, the baffles being arranged to cause gas withdrawn by the fan from the reaction zone-through and along the heating zone and expelled from the heating zone, to follow a zig-zag path along the heating zone, from side-to-side across the track and across any train of trolleys on the track. This fan may have a cooling air feed to its inlet for cooling the hot gases passing through it. In other words, there may be a plurality of the transverse partitions in the heating zone on opposite sides of the path, the partitions in the heating zone on each side of the path being staggered with regard to the partitions in the heating zone on the opposite side of the path, thereby being arranged to encourage gas flowing along the length of the tunnel in the heating zone to follow a zig-zag path along the heating zone, from side to side across the path and across any solid material on supports on the path.
As indicated above, a particular feature of the kiln of the present invention the provision, in what can be regarded as the freeboard of the tunnel, above any train of trolleys in the tunnel, a plurality of baffles extending across the width of the tunnel between its side walls, and below its roof, a combustion chamber being defined below the roof and above these baffles, and these baffles acting to reduce gas flow rates and turbulence above the train, thereby to resist passage or diffusion of carbon dioxide downwardly from the combustion chamber to material on the trolleys, and to promote non-turbulent flow of gases produced in the reaction zone in a direction upwardly from the trolleys and into the combustion zone in the combustion chamber.
The invention extends also to a kiln for the treatment of solid material undergoing an endothermic chemical reaction, the kiln including:
a horizontally extending tunnel having a hollow interior with an inlet end and an outlet end, the tunnel having a roof, a floor and a pair of opposed side walls;
a path for supports loaded with solid material to pass along in the interior of the tunnel in succession, from the inlet end of the kiln to the outlet end thereof, the path extending along the floor at the bottom of the interior of the tunnel from the inlet end of the kiln to the outlet end thereof; and
one or more heating surfaces for radiating radiant heat towards solid material loaded on supports passing along the path from said inlet end to said outlet end,
the kiln having a reaction chamber in the interior of the tunnel which is separated from the combustion chamber in the interior of the tunnel by at least one member of the group consisting of partitions, panels and baffles, the floor of the tunnel providing a floor for the reaction chamber along which reaction chamber floor the path for the supports extends.